Alkylating agents, and particularly methylating agents, are an important feedstock in the production of alkyl aromatics such as xylenes by alkylation. Of the xylene isomers, para-xylene (often abbreviated PX) is of particular value for many reasons, including its use in the manufacture of terephthalic acid, which is an intermediate in the manufacture of synthetic fibers. Equilibrium mixtures of xylene isomers either alone or in further admixture with ethylbenzene generally contain only about 24 wt % para-xylene and separation of p-xylene from such mixtures has typically required absorption and/or multistage refrigeration steps. Such processes have involved high operation costs and resulted in only limited yields.
Recently, an improved process of toluene methylation by methylating agents has been developed using certain aluminosilicate zeolite molecular sieve catalysts that, when treated and used under appropriate conditions, may exhibit high selectivity to the para-xylene isomer in toluene methylation, with per-pass toluene conversion of at least about 15%. This important development has been described in numerous patents and publications, such as U.S. Pat. Nos. 4,002,698; 4,356,338; 4,423,266; 5,675,047; 5,804,690; 5,939,597; 6,028,238; 6,046,372; 6,048,816; 6,156,949; 6,423,879; 6,504,072; 6,506,954; 6,538,167; and 6,642,426, the entirety of each of which is incorporated herein by reference.
However, some molecular sieve catalysts that may be utilized in efficient aromatic alkylation, such as toluene methylation with high selectivity to para-xylene, may be sensitive to various contaminants that can “poison” the catalyst (i.e., reduce the activity of the catalyst and/or reduce its selectivity to a desired end product). Poisoning can shorten catalyst life, requiring more frequent regeneration and/or replacement, significantly adding to the costs of an aromatic alkylation process employing the catalyst.